static cmos circuits -...
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ECE 261 Krish Chakrabarty 1
Static CMOS Circuits
• Conventional (ratio-less) static CMOS– Covered so far
• Ratio-ed logic (depletion load, pseudo nMOS)
• Pass transistor logic
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Example 1
module mux(input s, d0, d1,
output y);
assign y = s ? d1 : d0;
endmodule
1) Sketch a design using AND, OR, and NOT gates.
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Example 1module mux(input s, d0, d1,
output y);
assign y = s ? d1 : d0;
endmodule
1) Sketch a design using AND, OR, and NOT gates.
ECE 261 Krish Chakrabarty 4
Example 2
2) Sketch a design using NAND, NOR, and NOT gates. Assume ~S is available.
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Example 2
2) Sketch a design using NAND, NOR, and NOT gates. Assume ~S is available.
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Bubble Pushing• Start with network of AND / OR gates
• Convert to NAND / NOR + inverters
• Push bubbles around to simplify logic– Remember DeMorgan’s Law
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Example 3
3) Sketch a design using one compound gate and one NOT gate. Assume ~S is available.
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Example 3
3) Sketch a design using one compound gate and one NOT gate. Assume ~S is available.
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Compound Gates• Logical Effort of compound gates
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Example 4• The multiplexer has a maximum input capacitance
of 16 units on each input. It must drive a load of 160 units. Estimate the delay of the NAND and compound gate designs.
H = 160 / 16 = 10
B = 1
N = 2
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NAND Solution
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NAND Solution
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Compound Solution
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Compound Solution
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Example 5• Annotate your designs with transistor sizes that
achieve this delay.
Informal homework exercise (see textbook)!
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Input Order
• Our parasitic delay model was too simple– Calculate parasitic delay for Y falling
• If A arrives latest?
• If B arrives latest?
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Input Order
• Our parasitic delay model was too simple– Calculate parasitic delay for Y falling
• If A arrives latest? 2
• If B arrives latest? 2.33
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Inner & Outer Inputs
• Outer input is closest to rail (B)
• Inner input is closest to output (A)
• If input arrival time is known– Connect latest input to inner terminal
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Asymmetric Gates
• Asymmetric gates favor one input over another• Ex: suppose input A of a NAND gate is most critical
– Use smaller transistor on A (less capacitance)– Boost size of noncritical input– So total resistance is same
• gA = 10/9• gB = 2• gtotal = gA + gB = 28/9• Asymmetric gate approaches g = 1 on critical input• But total logical effort goes up
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Symmetric Gates
• Inputs can be made perfectly symmetric
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Skewed Gates• Skewed gates favor one edge over another
• Ex: suppose rising output of inverter is most critical– Downsize noncritical nMOS transistor
• Calculate logical effort by comparing to unskewed inverter with same effective resistance on that edge.– gu =
– gd =
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Skewed Gates• Skewed gates favor one edge over another• Ex: suppose rising output of inverter is most critical
– Downsize noncritical nMOS transistor
• Calculate logical effort by comparing to unskewed inverter with same effective resistance on that edge.– gu = 2.5 / 3 = 5/6– gd = 2.5 / 1.5 = 5/3
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HI- and LO-Skew
• Def: Logical effort of a skewed gate for a particular transition is the ratio of the input capacitance of that gate to the input capacitance of an unskewed inverter delivering the same output current for the same transition.
• Skewed gates reduce size of noncritical transistors– HI-skew gates favor rising output (small nMOS)
– LO-skew gates favor falling output (small pMOS)
• Logical effort is smaller for favored direction
• But larger for the other direction
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Catalog of Skewed Gates
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Catalog of Skewed Gates
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Catalog of Skewed Gates
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Asymmetric Skew
• Combine asymmetric and skewed gates– Downsize noncritical transistor on unimportant input
– Reduces parasitic delay for critical input
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Best P/N Ratio
• We have selected P/N ratio for unit rise and fall resistance (μ = 2-3 for an inverter).
• Alternative: choose ratio for least average delay
• Ex: inverter– Delay driving identical inverter
– tpdf =
– tpdr =
– tpd =
– Differentiate tpd w.r.t. P
– Least delay for P =
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Best P/N Ratio
• We have selected P/N ratio for unit rise and fall resistance (μ = 2-3 for an inverter).
• Alternative: choose ratio for least average delay
• Ex: inverter– Delay driving identical inverter
– tpdf = (P+1)
– tpdr = (P+1)(μ/P)
– tpd = (P+1)(1+μ/P)/2 = (P + 1 + μ + μ/P)/2
– Differentiate tpd w.r.t. P
– Least delay for P =
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P/N Ratios
• In general, best P/N ratio is sqrt of that giving equal delay.– Only improves average delay slightly for inverters
– But significantly decreases area and power
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Observations
• For speed:– NAND vs. NOR
– Many simple stages vs. fewer high fan-in stages
– Latest-arriving input
• For area and power:– Many simple stages vs. fewer high fan-in stages
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Combinational vs. Sequential Logic
Logic
CircuitOutIn
(a) Combinational
Output = f(In)
Logic
Circuit
OutIn
(b) Sequential
State
Output = f(In, Previous In)
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At every point in time (except during the switching transients) each gate output is connected to either VDD or Vss via a low-resistive path.
The outputs of the gates assume at all times the value of the Boolean function, implemented by the circuit (ignoring, once again, the transient effects during switching periods).
This is in contrast to the dynamic circuit class, which relies on temporary storage of signal values on the capacitance of high impedance circuit nodes.
Static CMOS Circuit (Review)
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Static CMOS (Review) VDD
VSS
PUN
PDN
In1
In2
In3
F =G
In1In2In3
PUN and PDN are Dual Networks
PMOS Only
NMOS Only
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Properties of Complementary CMOS Gates (Review)
High noise margins:VOH and VOL are at VDD and GND, respectively.
No static power consumption:There never exists a direct path between VDD andVSS (GND) in steady-state mode.
Comparable rise and fall times:(under the appropriate scaling conditions)
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Influence of Fan-In and Fan-Out on Delay
VDD
A B
A
B
C
D
C D
tp a1FI a2FI 2 a3FO+=
Fan-Out: Number of Gates Connected
FanIn: Quadratic Term due to:
1. Resistance Increasing2. Capacitance Increasing
+
Every fanout (output) adds two gate capacitances (pMOS and nMOS)
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Fast Complex Gate-Design Techniques• Trans is tor Sizing :
As long as Fan-out Capacitance dominates
• Progres s ive Sizing :
CL
In1
InN
In3
In2
Out
C1
C2
C3
M1 > M2 > M3 > MN
M1
M2
M3
MN
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Fast Complex Gate - Design Techniques
In1
In3
In2
C1
C2
CL
M1
M2
M3
In3
In1
In2
C3
C2
CL
M3
M2
M1
(a) (b)
• Trans is tor Ordering
critical pathcritical path
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Fast Complex Gate - Design Techniques
• Improved Log ic Des ign
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Ratioed LogicVDD
VSS
PDNIn1In2In3
F
RLLoad
VDD
VSS
In1In2In3
F
VDD
VSS
PDNIn1In2In3
F
VSS
PDN
Resistive DepletionLoad
PMOSLoad
(a) resistive load (b) depletion load NMOS (c) pseudo-NMOS
VT < 0
Goal: to reduce the number of devices over complementary CMOS
Careful design needed!
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Ratioed LogicVDD
VSS
PDN
In1In2In3
F
RLLoad
Resistive
RPDN
• VOH = VDD
VOL = RPDN
RL + RPDN
Desired: RL >> RPDN (to keep noise margin low)
tPLH = 0.69RLCL
Problems: 1) Static power dissipation
2) Difficult to implement a large resistor, eg 40k resistor (typical value) needs 3200 μ2 of n-diff, i.e. 1,000 transistors!
VDD
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Active LoadsVDD
VSS
In1In2In3
F
VDD
VSS
PDNIn1In2In3
F
VSS
PDN
DepletionLoad
PMOSLoad
depletion load NMOS pseudo-NMOS
VT < 0
• Depletion-mode transistor has negative threshold• On if VGS = 0• Body effect may be a problem!
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Pseudo-nMOSVDD
A B C D
FCL
• No problems due to body effect• N-input gate requires only N+1 transistors• Each input connects to only a single transistor, presenting smaller load to preceding gate• Static power dissipation (when output is zero)• Asymmetric rise and fall times
Example: Suppose minimal-sized gate consumes 1 mW of static power. 100, 000 gate-circuit: 50 W of static power (plus dynamic power)! (half the gates are in low-output state)• Effective only for small subcircuits where speed is important, eg address decoders in memories
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Pseudo-NMOS NAND GateVDD
GND
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Pass-Transistor Logic
Switch
Network
OutOut
A
B
B
B
• No s tatic cons umption
Inputs
AND gate
Is this transmission gatesnecessary?
Need a low impedance path to ground when B = 0
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Pass-Transistor Based Multiplexer
GND
VDD
In1 In2S S
S S
Out F
F = AS + BS
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Transmission Gate XOR
A
B
F
B
A
B
B
M 1
M 2
M 3 / M 4
6 transistors only!
Case 1:B = 1, M3/M4 turnedoff, F = AB
Case 2:B = 0, M3/M4 turned on, F = AB
F always has a path to VDD or Gnd, hence low impedance nodeIf not, node would be dynamic, requiring refresh due to charge leakage
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Delay in Transmission Gate Networks
V 1 V i - 1
C
5 5
0 0
V i V i + 1
C C
5
0
V n - 1 V n
C C
5
0
I n
V 1 V i V i + 1
C
V n - 1 V n
C C
I n R e q R e q R e q R e q
C C
( a )
( b )
C
R e q R e q
C C
R e q
C C
R e q R e q
C C
R e q
C I n
m
( c ) Insert buffers after every m switches
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Delay in Transmission Gate Networks
Consider Kirchoff’s Law at node Vi
Vi+1-Vi + Vi-1-Vi C dVi
Req Reqdt=
Therefore, dVi Vi+1 + Vi-1 - 2Vi
dt ReqC=
Propagation delay can be determined using Elmore delay analysis
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Delay Optimization
Delay can be reduced by adding buffers after m stages (tbuf = delay of a buffer)
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Transmission Gate Full Adder
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NMOS Only Logic: Level Restoring Transistor
M2
M1
Mn
Mr
OutA
B
VDDVDDLevel Restorer
X
• Advantage: Full Swing
• Disadvantage: More Complex, Larger Capacitance
• Other approaches: reduced threshold NMOS
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Single Transistor Pass Gate with VT=0
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Complimentary Pass Transistor Logic
A
B
A
B
B B B B
A
B
A
B
F=AB
F = AB
F=A+B
F = A+B
B B
A
A
A
A
F=A
F = A
OR/NOR EXOR/NEXOR AND/NAND
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F
F
Pass-Transistor Network
Pass-Transistor Network
A A B B
A A B B
Inverse
(a)
(b)
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4 Input NAND in CPL
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